1. Field of the Invention
The present invention relates to an image forming apparatus for copiers, facsimiles and the like, and more specifically relates to an image forming apparatus including a laminate image-bearing member comprising a dielectric layer or photoconductive layer superimposed on an electrically conductive substrate, a transfer member for making pressure contact with the image-bearing member, and a cleaning member for cleaning the surface of the image-bearing member through contact therewith, in which an electrostatic latent image is formed on the image-bearing member, and is developed into a visible toner image, and the visible toner image is transferred from the image-bearing member to a transfer sheet passing through a contact-pressure portion between said image-bearing member and a transfer member by applying a constant-current regulated transfer bias voltage to the transfer member.
2. Description of the Related Art
An image forming apparatus provided with a laminate image-bearing member, comprising a dielectric layer or photoconductive layer superimposed on an electrically conductive substrate, and with a transfer member, which comes into pressure contact with the image-bearing member, an electrostatic latent image is formed on the surface of the image-bearing member, and the electrostatic latent image is developed into a visible toner image, which is transferred from said image-bearing member to a transfer sheet while passing through the pressure contact surfaces of the image-bearing member and transfer member, i.e., the contact-pressure nip portion between the image-bearing member and the transfer member, by applying a constant-current regulated transfer bias voltage to said transfer member, and is fixed on the transfer sheet by a fixing means to produce an image.
An example of such an image forming apparatus is described hereinafter with reference to FIG. 4. The image forming apparatus of FIG. 4 is a printer, and is provided with a photosensitive drum 1 rotatably driven in the direction of arrow "a" by a drive means not shown in the drawing. Arranged sequentially around the periphery of photosensitive drum 1 are a charger 11, a developing device 12, a transfer roller 2, a cleaning blade 3, and an eraser 13. An image exposure unit 14 is arranged above photosensitive drum 1. A timing roller 15, is provided on one side of transfer roller 2, and a transfer sheet transport deck 16 and fixing roller 17 are, respectively, provided on the other side of transfer roller 2.
When printing, the surface of photosensitive drum 1 is uniformly charged by charger 11, and an electrostatic latent image is formed on the charged region via optical exposure of the drum surface by laser light L emitted from image exposure unit 14 in accordance with a document image. The formed latent image is developed by developing device 12 so as to form a visible toner image. In addition, a transfer sheet S is fed from a supply unit not shown in the drawing, and is transported via timing roller 15 to a transfer section synchronously with the toner image formed on the surface of photosensitive drum 1. In the transfer section, a transfer bias voltage is supplied from transfer bias power source 21 to transfer roller 2 so as to transfer the toner image onto transfer sheet S. After the toner image is transferred, transfer sheet S is transported to fixing roller 17 via transport deck 16, whereupon the toner image is fixed onto transfer sheet S and sheet S is subsequently discharged from the apparatus. Residual toner remaining on the surface of photosensitive drum 1 is removed therefrom by cleaning blade 3, and the residual charge on said drum surface is eliminated by eraser 13.
In the aforesaid image forming apparatus, the method of applying a transfer bias to a contact type transfer member such as transfer roller 2 may be a constant-voltage regulation method, or a constant-current regulation method. In the constant-voltage regulation method, the transfer current applied to the transfer sheet changes with differences in the thickness of the transfer sheet and varies depending on the type of sheet, i.e., when the electrical resistance of the transfer sheet changes, it influences image transfer effectiveness. Compared to transfer insufficiency which occurs in the aforesaid method, in the constant-current regulation method, a voltage is applied which is adjusted to the thickness of various transfer sheets, such that suitable image transfer is achieved without fluctuation in transfer efficiency because a constant transfer current normally results.
However, when a constant-current regulation method is used, and the edge portion of the transfer member 2 exceeds the width of the transfer sheets because of the width dimension of the transfer sheet, and the edge portion of said transfer member makes direct pressure contact with the image-bearing member 1, in the portion of the transfer member in direct pressure contact with the image-bearing member, a significant current flows past the transfer sheet with the result that the voltage required for transfer is reduced, thereby increasing the aforesaid transfer insufficiency.
Japanese Unexamined Patent Application No. HEI2-272590, for example, discloses a transfer method wherein the transfer current supplied by a constant-current regulated transfer bias changes in accordance with the width of a transfer sheet, such that a uniform charge is applied per unit area of the transfer sheet regardless of the width of said transfer sheet, thereby achieving a suitable transfer.
When a constant-current regulation method is used, the load resistance changes depending on the type of transfer sheet, e.g., an overhead projector (OHP) transparency, plain paper transfer sheet and the like, and the applied transfer voltage differs even though the transfer current is identical. FIG. 5 shows an example of changes in transfer characteristics (transfer efficiency), and voltage/current characteristics by type of transfer sheet in the laser printer of FIG. 4 when the system speed (circumferential speed of photosensitive drum 1) is set at 38 m/sec. In FIG. 5, "noiji 80T" and "OHP" represent types of transfer sheets. Reference label IW represents characteristics when a white document (no toner present) is used, and reference label IB represents characteristics when a solid image document is used. Reference label IPC represents the voltage/current characteristics without a transfer sheet when the current flows directly to photosensitive drum 1. As can be understood from FIG. 5, the applied transfer voltage differs even when the optimum transfer current is identical.
To eliminate the previously described problem, Japanese Unexamined Patent Application No. HEI2-264987, for example, discloses a transfer method wherein the transfer current of a constant-current regulated transfer bias changes in accordance with the type of transfer sheet so as to thereby suitably regulate a transfer voltage to achieve better transfer.
The disadvantages described below occur even when the transfer voltage of a constant-current regulated transfer bias is changed in accordance with the type and width of a transfer sheet as previously mentioned.
When, for example, a cleaning means for cleaning residual toner remaining on the surface of the image-bearing member after a toner image is transferred to a transfer sheet, such as a cleaning blade, cleaning brush or the like, comes into contact with the surface of the image-bearing member, the dielectric layer or photoconductive layer of the image-bearing member becomes gradually worn after repeated image formations, and the thickness of the layer is reduced by a considerable amount. The change in dielectric layer thickness occurs not only by the cleaning means but also through contact with the transfer member and transfer sheet. When the voltage applied to the transfer member is constant, the transfer current increases as the aforesaid layer thickness is reduced. FIG. 6 shows the change in optimum transfer current relative to change in the layer thickness of the photosensitive member in the condition wherein a transfer voltage of 1 kV is applied to transfer roller 2 in the laser printer of FIG. 4.
Accordingly, the previously described disadvantages do not result from changes in the transfer current in accordance with transfer sheet width or type of transfer sheet, and when the layer thickness of the photosensitive member changes, the previously set optimum current value changes and is not the desired value for image transfer, thereby resulting in transfer insufficiency.
In addition, when the edge portion of a transfer member exceeds the width of a transfer sheet, or medium, and makes direct pressure contact with the image-bearing member, a change in the ratio of the resistance value at the location in direct contact and the resistance value at the location in contact with the transfer sheet occurs, such that the current flowing through the location in direct contact increases, and the transfer current flowing through the transfer sheet decreases so as to cause at times obvious transfer insufficiency.
In order to eliminate the previously described disadvantages, an object of the present invention is to provide an image forming apparatus capable of accomplishing suitable image transfer without regard to changes in layer thickness of the dielectric layer or photoconductive layer of an image-bearing member.